1. Field of the Invention
This invention relates generally to the field of treating liquid radioactive waste, such as that produced by nuclear power plants. More particularly, the invention concerns a two-stage process for removing suspended and dissolved solids from low-level radioactive waste streams for permanent disposal. The process provides reusable water, while reducing the overall volume of the removed solid materials.
2. Background Art
In the nuclear power industry, the treatment of liquid radioactive waste is a very important concern. A typical nuclear power plant processes on the order of 10-20 million gallons of radioactive contaminated waste per year, from a number of different sources. The waste water typically includes process leakage water, water from process drains, water used to flush radioactive systems, and rain water leakage. To further complicate matters, the nature of the solid contaminants in the waste can vary greatly from one power plant to another.
If nuclear power is to continue as a viable energy option, the nuclear power industry must be able to efficiently and economically treat the liquid waste that it generates, so that the water in the waste can be reused, or disposed of in an economical manner. The industry must also be able to dispose of the removed solid contaminants in a safe, efficient manner. Since the cost of disposing of a given volume of solid waste is increasing greatly as more and more disposal sites are shut down, the volume of the solid waste must be decreased as much as possible before disposal.
Depending on the solids content of the liquid waste, two primary forms of treatment have typically been used. For liquid waste having a low solids content, filtration and reverse osmosis have been the preferred approach. For liquid waste having a high solids content, thermal evaporation has been the preferred approach. However, each of these approaches has significant drawbacks. For instance, the use of ion exchange demineralizers requires the disposal of large volumes of ion exchange resin or regeneration solution. Thermal evaporators, on the other hand, are very expensive to construct and operate. Evaporators are also very energy intensive, especially where the waste has a very low solids content. This can greatly increase customer power consumption costs. Evaporators have also experienced problems with heat transfer surface corrosion, which leads to expensive repairs and high radiation exposure to personnel.
Another approach has been to combine these various operations. For instance, U.S. Pat. No. 4,105,556 (D'Amaddio et al.) discloses an apparatus in which liquid radioactive waste is treated by filtration and reverse osmosis before introduction into an evaporator. However, in the system disclosed by D'Amaddio, all of the treatment steps are carried out at a single location, as part of an integrated continuous process. If the treatment is carried out at the power plant where the waste was generated, then the plant must have its own complete evaporator facility. On the other hand, if the treatment is carried out at a remote site, a great deal of effort and expense goes towards shipping large volumes of waste that is mostly water. Moreover, once the radioactive contaminants have been removed from the water at the remote site, the water must be shipped back to the plant to be reused.
In light of these and other deficiencies in prior art waste treatment systems, there is a need for a method of treating liquid radioactive waste that will provide reduced volumes of radioactive solids for disposal, while providing reusable water. There is also a need for a system of treating liquid radioactive waste that does not require each power station to have a costly evaporation system. There is also a need for a system for treating liquid radioactive waste that does not require the shipment of large amounts of liquid to a site where a thermal evaporator is available. These and a number of other objects are achieved by the present invention.